Oxide ion conductor and method of producing the same

ABSTRACT

A lanthanum oxide (La 2 O 3 ) powder, a germanium oxide (GeO 2 ) powder, and a strontium carbonate (SrCO 3 ) powder are mixed in a ratio so that a composition of the obtained composite oxide La l X m (AO 4 ) 6−n (ZO 4 ) n O p  satisfies 8≦l+m&lt;10, 0≦m≦2, 0≦n≦2 and 0≦p≦2. Thenafter, the materials are formed and sintered to prepare an oxide ion conductor. The crystalline structure of La l X m (AO 4 ) 6−n (ZO 4 ) n O p  belongs to the apatite type structure. The conduction of oxide ion occurs when O 2−    14  occupying the  2   a  site of the apatite type structure moves along the c-axis direction.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an oxide ion conductor and amethod of producing the same. In particular, the present inventionrelates to an oxide ion conductor which is preferably usable as a solidelectrolyte of a fuel cell and which is composed of a composite oxide oflanthanum, a tetravalent element, and an element with which at least anyone of them is substituted. The present invention also relates to amethod of producing the oxide ion conductor.

[0003] 2. Description of the Related Art

[0004] The solid, in which ions are movable, is widely known as an ionconductor. Also, as well known, the ion has the positive or negativeelectric charge. Therefore, a current flows through the ion conductorwhen the ions move.

[0005] The mobile ion differs depending on the type of the ionconductor. For example, β-Al₂O₃, which has a composition ofNa₂O.11Al₂O₃, is a good ion conductor for Na⁺, i.e., a sodium ionconductor, which is adopted as a solid electrolyte of the sodium/sulfurcell. On the other hand, AgI has long been known as a good silver ionconductor from old times.

[0006] Recently, because of the growing concern over environmentalprotection, a fuel cell is used as a low pollution electric powersource. Attempts are being made to adopt an oxide ion (O²⁻) conductor asan electrolyte of the fuel cell. In this case, the entire fuel cell canbe made of solid materials, for the oxide ion conductor itself is asolid, making the structure simple. Further, the number of times ofmaintenance can be reduced because no liquid leakage occurs.

[0007] A typical crystalline structure of the oxide ion conductor is thefluorite (CaF₂) type structure. Examples of such structure includestabilized ZrO₂ doped with about 8 mole % of Y₂O₃, stabilized ZrO₂ dopedwith about 15 mole % of MgO, Bi₂O₃ doped with about 25 mole % of Y₂O₃,and CeO₂ doped with about 25 mole % of Gd₂O₃. In particular, the twotypes of stabilized ZrO₂ described above are practically used as a solidelectrolyte of a fuel cell and a partition wall of an oxygen sensor formeasuring oxygen concentration in gases or molten metals.

[0008] Examples of other oxide ion conductors are those which have theperovskite (CaTiO₃) type structure; for example,La_(0.9)Sr_(0.1)Ga_(0.8)Mg_(0.2)O₃ and BaTh_(0.9)Gd_(0.1)O₃. Usage ofthese compounds as a partition wall, an oxygen sensor and also athermistor, is being considered.

[0009] The oxide ion conductivity of the oxide ion conductor having thefluorite or the perovskite type structure described above issatisfactory at a high temperature range of about 800 to 1000° C.Therefore, to operate a fuel cell using the oxide ion conductor as asolid electrolyte, it is necessary to raise the temperature of alaminated stack to about 800 to 1000° C. The laminated stack comprises apower generation cell or a plurality of power generation cellselectrically connected to one another to constitute the fuel cell. At atemperature lower than the foregoing temperature range, the conductivityof the sold electrolyte (oxide ion conductor) decreases, thus, markedlylowering the power generation efficiency.

[0010] When, however, the fuel cell is operated at a high temperaturerange as described above, a large amount of energy (electric power orthe like) is required to heat the power generation cell or the laminatedstack. Additionally, inexpensive metal materials, such as stainlesssteel cannot be used as a member for constituting the fuel cell. Themechanical strength and the corrosion resistance of such a metalmaterial decreases at a high temperature. Therefore, operating the fuelcell at a high temperature significantly increases the running cost.

[0011] Both of Japanese Laid-Open Patent Publication Nos. 8-208333 and11-71169 suggest an oxygen ion conductor composed of a composite oxideof rare earth element(s) and Si, and a method of producing the same. Inboth of the patent documents, it is described that the composite oxideexhibits excellent oxide ion conductivities at low and middletemperature range of 200 to 600° C., as compared with the two types ofthe oxide ion conductors described above.

[0012] Further, in Solid State Ionics 136-137 (2000 edition), pp. 31-37,studies are made on La₁₀Si₆O₂₇, La₁₀Ge₆O₂₇ and composite oxides obtainedby substituting a part of La with Sr of the two. It is reported thatthese composite oxides also exhibit excellent oxide ion conductivitiesin the temperature range of 200 to 600° C. as compared with the twotypes of the oxide ion conductors described above.

[0013] Generally, to produce an oxide ion conductor composed of acomposite oxide of a rare earth element and Si, the composite oxide ofrare earth elements and Si are sintered at a temperature exceeding 1700°C. This is because the melting point of the foregoing composite oxide ishigh, and the sintering process is insufficient at a temperature lowerthan 1700° C. In other words, it is difficult to obtain a sinteredproduct (oxide ion conductor) at a temperature lower than 1700° C.,which is strong enough for practical use.

[0014] When, however, parts of the reactor, which are used forsintering, such as the heating element, heat insulating material andreaction tube, are heated up to a temperature exceeding 1700° C., thedurability rapidly falls. That is, the life of the reactor isdrastically shortened and the equipment cost is extremely expensive forproducing the oxide ion conductor composed of the composite oxide ofrare earth elements and Si. Consequently, the expensive production costof the oxide ion conductor is a great drawback.

[0015] In the scientific paper described above, La₁₀Ge₆O₂₇ andLa_(10−x)Sr_(x)Ge₆O₂₇, in which a part of La is substituted with Sr, areobtained by isostatically pressing a mixed powder of GeO₂, SiO₂, andSrCO₃ at 275 Mpa, and then sintering in a temperature range of 1600 to1650° C.

[0016] However, large amounts of impurities such as La₂GeO₅ and La₂Ge₂O₇are contained in the products of La₁₀Ge₆O₂₇ and La_(10−x)Sr_(x)Ge₆O₂₇obtained from above described procedure. The oxide ion conductivities ofthe oxide ion conductors containing such impurities are extremely low atlow through middle range temperatures as compared with the oxide ionconductivity of the pure oxide ion conductor. The conductivity of theoxide ion cannot be improved, because the impurities in the compositeoxides of lanthanum and germanium having the same compositions asdescribed above.

SUMMARY OF THE INVENTION

[0017] A principal object of the present invention is to provide anoxide ion conductor, and a method of producing the same, which isexcellent in oxide ion conductivity at a middle temperature range of 500to 700° C. Such an oxide ion conductor makes it possible to lower theoperating temperature for a fuel cell or the like.

[0018] Another object of the present invention is to provide an oxideion conductor composed of a composite oxide containing:

[0019] lanthanum, a tetravalent element A, and at least one of adivalent or tetravalent element X with which the lanthanum issubstituted and a trivalent or pentavalent element Z with which theelement A is substituted,

[0020] wherein the composite oxide has a composition formula which isrepresented by La_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2, and wherein the composite oxide has acrystalline structure which belongs to an apatite type structure.

[0021] The oxide ion conductor according to the present invention may beeither La_(l)X_(m)(AO₄)₆O_(p) (s=l+m) in which a part of La ofLa_(s)(AO₄)₆O_(p) (8≦s<10, 0≦p≦2) is substituted with the element X, orLa_(s)(AO₄)_(6−n)(ZO₄)_(n)O_(p) (in this case 0<n≦2 because of m=0) inwhich a part of the element A is substituted with the element Z.Alternatively, the oxide ion conductor according to the presentinvention may be La_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) in which a part ofLa is substituted with the element X, and a part of the element A issubstituted with the element Z. In the present invention, as describedabove, m and n are not simultaneously zero, which applies to allembodiments.

[0022] Still another object of the present invention is to provide amethod of producing an oxide ion conductor, comprising the steps of:

[0023] obtaining a mixed powder in which a lanthanum compound, acompound of a tetravalent element A, and at least one of a compound of adivalent or tetravalent element X and a compound of a trivalent orpentavalent element Z, are mixed in a ratio to produceLa_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that 8≦l+m<10, 0≦m≦2,0≦n≦2, 0≦p≦2;

[0024] forming the mixed powder to obtain a formed product; and

[0025] sintering the formed product to obtain the oxide ion conductorhaving an apatite crystalline structure and a composition formula whichis represented by La_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2.

[0026] Still another object of the present invention is to provide amethod of producing an oxide ion conductor, comprising the steps of:

[0027] obtaining a mixed powder in which a lanthanum compound, acompound of a tetravalent element A, and at least one of a compound of adivalent or tetravalent element X and a compound of a trivalent orpentavalent element Z are mixed in a ratio to produceLa_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that 8≦l+m<10, 0≦m≦2,0≦n≦2, 0≦p≦2;

[0028] heating the mixed powder to obtain a granular material of acomposite oxide of lanthanum and germanium having an apatite crystallineand a composition formula which is represented byLa_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that 8≦l+m<10, 0≦m≦2,0≦n≦2, 0≦p≦2;

[0029] pulverizing the granular material to obtain a composite oxidepowder;

[0030] forming the composite oxide powder to obtain a formed product;and

[0031] sintering the formed product to obtain the oxide ion conductorcomposed of the composite oxide. The granular material referred toherein is an aggregate of powder cohered or bonded so that the aggregateis capable of being pulverized in the pulverizing step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 shows a schematic structure of a unit lattice ofLa_(s)(GeO₄)₆O_(p) (8≦s<10, 0≦p≦2) as an oxide ion conductor;

[0033]FIG. 2 shows a flow chart illustrating a method (first productionmethod) for producing an oxide ion conductor according to a firstembodiment;

[0034]FIG. 3 shows a flow chart illustrating a method (second productionmethod) for producing an oxide ion conductor according to a secondembodiment;

[0035]FIG. 4 shows a table illustrating mixing ratios of raw materialpowders and compositions of obtained oxide ion conductors in Examples 1to 10 and Comparative Examples 1 and 2;

[0036]FIG. 5 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 1 to 5 and Comparative Examples 1 and 2;

[0037]FIG. 6 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 6 to 10 and Comparative Examples 1 and 2;

[0038]FIG. 7 shows a table illustrating mixing ratios of raw materialpowders and compositions of obtained oxide ion conductors in Examples 11to 14;

[0039]FIG. 8 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 11 and 12 and Comparative Examples 1 and 2;

[0040]FIG. 9 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 13 and 14 and Comparative Examples 1 and 2;

[0041]FIG. 10 shows a table illustrating mixing ratios of raw materialpowders and compositions of obtained oxide ion conductors in Examples 15to 18;

[0042]FIG. 11 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 15 and 16 and Comparative Examples 1 and 2;

[0043]FIG. 12 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 17 and 18 and Comparative Examples 1 and 2;

[0044]FIG. 13 shows a table illustrating mixing ratios of raw materialpowders and compositions of obtained oxide ion conductors in Examples 19to 22;

[0045]FIG. 14 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 19 and 20 and Comparative Examples 1 and 2; and

[0046]FIG. 15 shows a graph illustrating the relationship between theoxide ion conductivity and the temperature in relation to the oxide ionconductors of Examples 21 and 22 and Comparative Examples 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The oxide ion conductor and the method of producing the sameaccording to the present invention will be explained in detail belowwith reference to the accompanying drawings as exemplified by preferredembodiments.

[0048] At first, FIG. 1 shows the structure of the unit lattice ofLa_(s)(GeO₄)₆O_(p) (wherein 8≦s<10, 0≦p≦2) as viewed in the c-axisdirection. The unit lattice 10 has the apatite type structure includingsix GeO₄ tetrahedrons 12, O²⁻ 14 occupying the 2 a sites, and La³⁺ 16 a,16 b occupying the 4 f sites and the 6 h sites respectively. Ge⁴⁺ andO²⁻ of the GeO₄ tetrahedron 12 are not shown.

[0049] The crystal system of the unit lattice 10 belongs to thehexagonal system. In FIG. 1, angle α is formed at the intersection ofside AB in the a-axis direction and side BF is the c-axis direction ofthe unit lattice 10. Angle β is formed at the intersection of side BC inthe b-axis direction and side BF, and angle γ, at the intersection ofsides AB and BC. The angles α, β, γ are 90°, 90°, and 120° respectively.The length of side AB is equal to the length of side BC. Further, thelengths sides AB, BC differ from the length of side BF.

[0050] The hexagonal crystal lattice (not shown), in which the unitlattice 10 is included, is a simple lattice. When the hexagonal crystallattice makes a ⅓ rotation around a virtual screw axis (not shown), andtranslates along the screw axis by half the length of side BF, thepositions of each ion synchronizes exactly with the positions before thedisplacement. Further, the mirror plane of the hexagonal crystal latticeis perpendicular to the screw axis. Namely, when the space group of thecrystal of La_(s)(GeO₄)₆O_(p) (8≦s<10, 0≦p≦2) is symbolized by aHermann-Mauguin symbol P6₃/m.

[0051] An oxide ion conductor according to an embodiment of the presentinvention is a composite oxide containing an element with which at leastone of La³⁺ 16 a, La³⁺ 16 b and Ge in the GeO₄ tetrahedron 12 of theunit lattice 10 is substituted. That is, when X represents the elementexisting after the substitution of La³⁺ 16 a and/or La³⁺ 16 b and Zrepresents the element existing after the substitution of Ge therewith,the composition formula of the composite oxide isLa_(l)X_(m)(GeO₄)_(6−n)(ZO₄)_(n)O_(p). In this composition formula,8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2, and m and n are not simultaneously zero.

[0052] As described above, only one of the elements X, Z may be presentin the composite oxide. Therefore, the oxide ion conductor (compositeoxide) according to the embodiment of the present invention includesLa_(s)(GeO₄)_(6−n)(ZO₄)_(n)O_(p) (m=0), which does not contain elementX, and La_(l)X_(m)(GeO₄)₆O_(p) (n=0), which does not contain element Z.

[0053] The structure and the crystal system of the unit lattice of thecomposite oxide containing the elements X, Z as described above, andhave the apatite type structure and the hexagonal system like the unitlattice 10. Therefore, the space group of the crystal of the compositeoxide is also symbolized by a Hermann-Mauguin symbol P6₃/m.

[0054] The reason why La_(l)X_(m)(GeO₄)_(6−n)(ZO₄)_(n)O_(p) having theapatite type structure is an excellent oxide ion conductor, seems to bethat O²⁻ 14 occupying the 2 a site, does not bond with the GeO₄tetrahedron 12, La³⁺ 16 a or La³⁺ 16 a. Since the force exerted on O²⁻14 is not strong, O²⁻ 14 can move relatively freely in the c-axisdirection without being constrained to the 2 a site.

[0055] On the other hand, composite oxides containing La, Ge, X, and Zwhich do not have the apatite type crystalline structure, have low oxideion conductivities for the following reason. The unit lattice of such acomposite oxide has a different structure from that of the unit lattice10 described above. Therefore, O²⁻, which can move freely like O2− 14 inthe unit lattice 10, described above, do not exist.

[0056] In view of the above, in the oxide ion conductor according to theembodiment of the present invention, the sum of l and m inLa_(l)X_(m)(GeO₄)_(6−n)(ZO₄)_(n)O_(p) is set to be 8 or more but lessthan 10. If (l+m) is less than 8, the crystalline structure does notform the apatite type structure, and hence the oxide ion conductivity islowered. On the other hand, if (l+m) is not less than 10, compositeoxides having a different structure like impurities such as La₂GeO₅ arecontained in the composite oxide having the apatite type structure;therefore the oxide ion conductivity is also lowered.

[0057] In particular, it is preferable that 8≦l+m≦9.33 for the followingreason. In this state, a large majority ofLa_(l)X_(m)(GeO₄)_(6−n)(ZO₄)_(n)O_(p) crystals have the apatite typestructure (see FIG. 1), and hardly any impurity phase having any otherstructure is produced. Therefore, the oxide ion conductivity of thecomposite oxide is the heightened. Especially when the value of l+m is9.33, the oxide ion conductivity is the highest.

[0058] Further, the unit lattice of the crystal ofLa_(l)X_(m)(GeO₄)_(6−n)(ZO₄)_(n)O_(p) is slightly strained as comparedwith the unit lattice 10 of the crystal of La_(s)(GeO₄)₆O_(p), becauseof the presence of the element X or the element Z. Accordingly, thedistance between the 2 a sites of an arbitrary unit lattice of thecomposite oxide and a neighboring unit lattice is narrowed. Therefore,O²⁻ 14 occupying the 2 a site can move with ease in the c-axis directionbetween the unit lattices. Accordingly, the composite oxide exhibitsexcellent oxide ion conductivity.

[0059] The composite oxide exhibits excellent oxide ion conductivity ata middle temperature range of 500 to 700° C. Therefore, for example,with a fuel cell provided with the composite oxide as a solidelectrolyte according to the embodiment of the present invention,equivalent power generation characteristics are obtained even when thefuel cell is operated at a low temperature, as compared with theconventional fuel cell. Therefore, it is possible to decrease therunning cost of the fuel cell.

[0060] The element X is not specifically limited provided that theelement X is a divalent or tetravalent element and has an ionic radiuscapable of maintaining the apatite type structure when La³⁺ 16 a and/orLa³⁺ 16 b is substituted. Preferred examples of the element X include Sr(divalent) and Zr (tetravalent).

[0061] The element Z is not specifically limited provided that theelement Z is a trivalent or pentavalent element and has an ionic radiuscapable of maintaining the apatite type structure when Ge⁴⁺ issubstituted. Preferred examples of the element Z include Al (trivalent)and P (pentavalent).

[0062] Next, the method of producing the composite oxide will beexplained.

[0063] A method of producing the composite oxide according to a firstembodiment (referred to as “first production method”) will be explainedwith reference to a flow chart shown in FIG. 2 in which Sr is selectedas the element X and strontium carbonate is used as an Sr source toprepare La_(l)Sr_(m)(GeO₄)₆O_(p). The first production method comprisesa step S1 of mixing a lanthanum oxide powder, a germanium oxide powder,and a strontium carbonate powder to obtain a mixed powder, a step S2 offorming the mixed powder to obtain a formed product, and a step S3 ofsintering the formed product to produce the composite oxide (sinteredproduct).

[0064] Firstly, in step S1, the lanthanum oxide (La₂O₃) powder, thegermanium oxide (GeO₂) powder, and the strontium carbonate (SrCO₃)powder are mixed.

[0065] The ratio of the La₂O₃ powder, the GeO₂ powder, and the SrCO₃powder is set so that the crystals of La_(l)Sr_(m)(GeO₄)₆O_(p) have theapatite type structure; in other words, the values of l, m, p satisfy8≦l+m<10, 0<m≦2, 0≦p≦2. For example, when producing a composite oxidehaving a composition represented by La_(8.93)Sr_(0.1)(GeO₄)₆O_(1.), thefollowing ratio is set; La₂O₃ powder:GeO₂ powder:SrCO₃ powder=44.65:60:1(numerals are expressed by molar ratios).

[0066] Secondly, in step S2, the mixed powder is formed. The formingmethod in this process is not limited to any specified method. It ispossible to adopt forming methods including, for example, by pressforming, slurry casting, and extruding. The shape of the formed productmay be of any shape depending on the use.

[0067] Subsequently, in step S3, the La₂O₃ powder, the GeO₂ powder, andthe SrCO₃ powder are subjected to grain growth by sintering the formedproduct. That is, the junctions of contacting particles or grains growto combine into large particles or grains. Further, La₂O₃, GeO₂, andSrCO₃ form the solid solution to obtain the composite oxide representedby La_(l)Sr_(m)(GeO₄)₆O_(p). Accordingly, the sintered product, i.e.,the composite oxide as the oxide ion conductor is produced.

[0068] It is preferable that the sintering temperature is within therange of 1400 to 1700° C. for the following reason. If the sinteringtemperature is less than 1400° C., the grain growth does not progressefficiently. On the other hand, if the temperature exceeds 1700° C., thedurability of the heating element, the heat insulating material, and thereaction tube of the reactor used for the sintering operation, isdrastically lowered. The sintering temperature range of 1450 to 1600° C.is preferred over the foregoing temperature range, and the mostpreferred temperature is 1500° C.

[0069] As described above, in the first production method, the sinteringtemperature, at which La_(l)Sr_(m)(GeO₄)₆O_(p) is obtained, is lowerthan the sintering temperature at which La₁₀Si₆O₂₇ as the oxide ionconductor in the conventional art is obtained. Accordingly, it ispossible for the reactor to have a long life span, and to reduce theproduction cost.

[0070] Further, because the ratios of the La₂O₃ powder, the GeO₂ powder,and the SrCO₃ powder are set as described above, the apatite typestructure is obtained, in which the crystals belong to the hexagonalsystem whose space group is symbolized by P6₃/m. Therefore,La_(l)Sr_(m)(GeO₄)₆O_(p), can exhibit excellent oxide ion conductivity.

[0071] The crystalline structure of La₂O₃, GeO₂, and SrCO₃ andLa_(l)Sr_(m)(GeO₄)₆O_(p) is different from each other. For example, thecrystalline structure of La₂O₃ differs greatly from the apatite typestructure. Therefore, as in the first production method, when the formedproduct is directly sintered to simultaneously enhance change of thecrystalline structure brought about by forming the solid solution ofLa₂O₃ and GeO₂ and the grain growth, any of the changes of thecrystalline structure and grain growth will not be enhancedsufficiently, if the driving force in the sintering process is small.Accordingly, as explained below, it is desirable that the step ofmutually forming the solid solution of La₂O₃ and GeO₂ is distinct fromthe step of affecting the grain growth.

[0072] A method of producing the composite oxide according to a secondembodiment (referred to as “second production method”) will be explainedwith reference to a flow chart shown in FIG. 3 in which Al is selectedas the element Z and aluminum oxide is used as an Al source to prepareLa_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p).

[0073] The second production method comprises a step S10 of mixing anLa₂O₃ powder, a GeO₂ powder, and an Al₂O₃ powder to obtain a mixedpowder, a step S20 of heat-treating the mixed powder to obtain agranular material of a composite oxide of lanthanum and germanium, astep S30 of pulverizing the granular material to obtain a compositeoxide powder, a step S40 of forming the composite oxide powder to obtaina formed product, and a step S50 of sintering the formed product toobtain the composite oxide.

[0074] Firstly, in the step S10, the La₂O₃ powder, the GeO₂ powder, andthe Al₂O₃ powder are mixed in accordance with the mixing step S1 of thefirst production method. In this procedure, the ratio of the respectivepowders are set so that the crystals ofLa_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) have the apatite type structure, inother words, the values of s, n, p satisfy 8≦s<10, 0<n≦2, 0≦p≦2. Forexample, when a composite oxide having a composition represented byLa₉(GeO₄)₅(AlO₄)O_(1.0) is obtained, the ratio is set to be La₂O₃powder:GeO₂ powder:Al₂O₃ powder=9:10:1 (numerals are expressed by molarratios).

[0075] Secondly, in step S20, the mixed powder is heated, and thus thepowders are fused so that the obtained product is capable of beingpulverized. In other words, the powders are aggregated or bonded toobtain a granular material so that the pulverization can be performed.If, at this point, the sintered product is dense, it is extremelydifficult to carry out the pulverization.

[0076] The heat treatment temperature in the step S20 is set so thatthere is no conspicuous grain growth of the mixed powder. In thisprocedure, the La₂O₃ powder, the GeO₂ powder, and the Al₂O₃ powder areused. Therefore, the temperature range is sufficient at 700 to 1200° C.,preferably 1000° C.

[0077] During this process, La₂O₃, GeO₂, and Al₂O₃ form a solidsolution. That is, at this point in the second production method,La_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) is produced. The heat treatment instep S20 is performed until the production ofLa_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) is completed. Specifically, the heattreatment may be performed for about 2 hours. The values of s, n, p, aregiven as 8≦s<10, 0<n≦2, 0≦p≦2.

[0078] When the heat treatment for the mixed powder is performed at theforegoing temperature range for the foregoing period of time, theaggregation force or the bonding force of the obtained granular materialof La_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) need not be so strong. The strengthis such that the granular material can be pulverized easily with amortar.

[0079] Subsequently, in step S30, the granular material ofLa_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) is pulverized to obtain a powder. Thepulverization method is not specifically limited. The pulverization maybe performed with a mortar; however, it is preferable to use a methodsuch as ball milling in which the particle diameter of the powder can besubstantially uniformed. Accordingly, hardly any pores remain in thesintered product, and it is possible to obtain an oxide ion conductorwith excellent strength and toughness.

[0080] Next, in step S40, a formed product is prepared in accordancewith step S2 of the first production method.

[0081] Finally, in step S50, the formed product is sintered inaccordance with step S3 to obtain the oxide ion conductor. Also in thesecond production method, the sintering temperature is preferably in therange of 1400 to 1700° C. and more preferably 1450 to 1600° C. as instep S3. The sintering temperature is most preferably 1500° C.

[0082] In the second production method, the step (step S20) of formingthe solid solution of La₂O₃, GeO₂, and Al₂O₃ and the step (step S50) ofaffecting the grain growth are performed separately. Therefore, thedriving force required for the structural change of the crystals and thedriving force required for the grain growth are decreased. Thus, it ispossible to obtain a composite oxide in whichLa_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) having the apatite type structure ishomogeneously produced within the entire product. That is, in thisprocedure, there is no decrease of the oxide ion conductivity, whichwould otherwise be caused by the production of any composite oxidehaving a structure other than the apatite type structure.

[0083] Further, when the granular material is pulverized in the step S30so that the particle diameters are substantially uniformed, the oxideion conductor will be made of a dense sintered product, even when theparticle diameter differs greatly among the La₂O₃ powder, the GeO₂powder, and the Al₂O₃ powder. Such a dense sintered product has astrength and toughness for sufficient practical use.

[0084] According to the first and second production methods as describedabove, La_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p), with the apatite crystallinestructure and hence the excellent oxide ion conductivity, is obtainedwith ease by performing the sintering operation after mixing the rawmaterial powders in the predetermined ratios and performing the formingoperation. Further, it is possible to reduce the production cost of theoxide ion conductor, because the sintering temperature will not exceed1700° C.

[0085] When an oxide ion conductor, which is composed ofLa_(l)Sr_(m)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) containing both Sr and Al, isprepared, then an La₂O₃ powder, a GeO₂ powder, an SrCO₃ powder, and anAl₂O₃ powder may be used as raw materials, and the respective powdersmay be mixed in a ratio so that crystals have the apatite typestructure.

[0086] In the first and second production methods described above, theforming steps S2, S40 and the steps S3, S50 are performed individually.However, the forming and the sintering may be performed simultaneouslyby adopting the hot press method or the hot isostatic pressing (HIP)method.

[0087] In the first and second production methods, the mixed powder isobtained by mixing the lanthanum oxide powder and the germanium oxidepowder. However, the mixed powder may be obtained by using a powder of asubstance other than oxide. For example, the carbonate of lanthanum andcarbonate of germanium can be used.

[0088] The mixed powder, which contains lanthanum, germanium, andoxygen, may be obtained by performing the sol-gel method, the CVDmethod, or the spray pyrolysis method. In this case, various conditionsmay be controlled to prepare a mixed powder withLa_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) (8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2).Also, a powder other than oxide may be prepared.

EXAMPLES

[0089] 34.75 g of a La₂O₃ powder, 14.9 g of a GeO₂ powder, and 0.35 g ofan SrCO₃ powder were mixed for 16 hours in a wet ball mill with a 100 gsolvent of ethyl alcohol. 500 g of balls made of ZrO₂ were used in theball mill. After that, the solvent was removed by using a rotaryevaporator to obtain a mixed powder.

[0090] Subsequently, a crucible made of Al₂O₃, in which the mixed powderwas accommodated, was arranged in a reaction furnace. The mixed powderwas heated in an atmospheric air at 1000° C. for 2 hours to obtain agranular material. The granular material was pulverized by using a wetball mill under the same condition as above.

[0091] The powder was formed into a disk member having a diameter of 12mm and a thickness of 3 mm by die pressing and the isostatic formingmethod. After that, the disk member was placed on a jig composed of astabilized ZrO₂ sintered member and arranged in a reaction furnace.Then, the disk member was sintered at 1500° C. for 2 hours in anatmospheric atmosphere to obtain an oxide ion conductor composed of asintered product of La_(8.93)Sr_(0.1)(GeO₄)₆O_(1.5) with the apatitecrystalline structure. This product is designated as Example 1.

[0092] Further, oxide ion conductors with various compositions wereobtained in accordance with Example 1, except that the La₂O₃ powder, theGeO₂ powder, and the SrCO₃ powder were mixed in weights shown in FIG. 4.These products are designated as Examples 2 to 10. FIG. 4 also showsrespective values of l, m, p of the oxide ion conductorsLa_(l)Sr_(m)(GeO₄)₆O_(p) of Examples 1 to 10.

[0093] For the purpose of comparison, 36.16 g of the La₂O₃ powder and7.99 g of the GeO₂ powder were mixed, and then an oxide ion conductorcomposed of a sintered product of La₁₀Ge₆O₂₇ was obtained in accordancewith Example 1. This product is designated as Comparative Example 1.Further, an oxide ion conductor composed of a sintered product ofLa₁₀Si₆O₂₇ was prepared. This product is designated as ComparativeExample 2. The sizes of the oxide ion conductors of Comparative Examples1 and 2 were set to be the same as those of the oxide ion conductors ofExamples 1 to 10.

[0094] The oxide ion conductivities of each oxide ion conductors inExamples 1 to 10 and Comparative Examples 1 and 2 described above weremeasured. That is, the sintered product was ground until its thicknesswas 2 mm, and then a Pt paste was applied to the both end surfaces sothat the diameter was 6 mm. Pt lead wires each having a diameter of 0.1mm were arranged on the Pt paste. Subsequently, the Pt paste wasretained and dried under a temperature of 120° C. for 1 hour, thenconnected by firing under 700° C. for 2 hours.

[0095] The Pt lead wires were connected to an Impedance Analyzer 4192Aproduced by Hewlett Packard, and the alternating current impedance wasmeasured at frequencies of 5 Hz to 13 MHz. The oxide ion conductivitywas calculated from the results of the measurement. Results are shown asfunctions of the temperature in FIGS. 5 and 6. FIG. 5 is illustrative ofthe comparisons between Examples 1 to 5 and Comparative Examples 1 and2, and FIG. 6 is illustrative of the comparisons between Examples 6 to10 and Comparative Examples 1 and 2.

[0096] According to FIGS. 5 and 6, it is clear that the oxide ionconductivities of the oxide ion conductors of Examples 1 to 10 arehigher than those of the oxide ion conductors of Comparative Examples 1and 2, when the temperature is not less than 500° C. It can be analyzedthat products, such as La₁₀Ge₆O₂₇ in which La is 10 and above, cannotretain the apatite type structure.

[0097] Next, oxide ion conductors having various compositions wereobtained in accordance with Example 1, except the La₂O₃ powder, the GeO₂powder, and a ZrO₂ powder were mixed in weights shown in FIG. 7. Theseproducts are designated as Examples 11 to 14 respectively. FIG. 7 alsoshows respective values of l, m, p of oxide ion conductorsLa_(l)Zr_(m)(GeO₄)₆O_(p) of Examples 11 to 14.

[0098] The oxide ion conductivities of the oxide ion conductors ofExamples 11 to 14 were calculated in the same manner as described above.Results are shown as functions of the temperature in FIGS. 8 and 9. FIG.8 is illustrative of the comparison between Examples 11 and 12 andComparative Examples 1 and 2. FIG. 9 is illustrative of the comparisonbetween Examples 13 and 14 and Comparative Examples 1 and 2. Accordingto FIGS. 8 and 9, it is clear that the oxide ion conductivities of theoxide ion conductors of Examples 11 to 13 are also higher than those ofthe oxide ion conductors of Comparative Examples 1 and 2, when thetemperature is not less than 500° C., and that the oxide ion conductorof Example 14 has the oxide ion conductivity substantially equivalent tothose of the oxide ion conductors of Comparative Examples 1 and 2.

[0099] Further, oxide ion conductors having various compositions wereobtained in accordance with Example 1 except that the La₂O₃ powder, theGeO₂ powder, and an Al₂O₃ powder were mixed in weights shown in FIG. 10.These products are designated as Examples 15 to 18 respectively. FIG. 10also shows respective values of s, 6−n, n, p of the oxide ion conductorsLa_(s)(GeO₄)_(6−n)(AlO₄)_(n)O_(p) of Examples 15 to 18.

[0100] Thenafter, the oxide ion conductivities of the oxide ionconductors of Examples 15 to 18 were calculated in the same manner asdescribed above. Results are shown as functions of the temperature inFIGS. 11 and 12. FIG. 11 is illustrative of the comparison betweenExamples 15 and 16 and Comparative Examples 1 and 2. FIG. 12 isillustrative of the comparison between Examples 17 and 18 andComparative Examples 1 and 2. According to FIGS. 11 and 12, it is clearthat the oxide ion conductivities of the oxide ion conductors ofExamples 15 to 18 are higher than those of the oxide ion conductors ofComparative Examples 1 and 2, when the temperature is not less than 500°C.

[0101] Further, oxide ion conductors having various compositions wereobtained in accordance with Example 1 except that the La₂O₃ powder, theGeO₂ powder, and an NH₄H₂PO₄ powder were mixed in weights shown in FIG.13. These products are designated as Examples 19 to 22. FIG. 13 alsoshows respective values of s, 6−n, n, p of the oxide ion conductorsLa_(s)(GeO₄)_(6−n)(PO₄)_(n)O_(p) of Examples 19 to 22.

[0102] Thenafter, the oxide ion conductivities of the oxide ionconductors of Examples 19 to 22 were calculated in the same manner asdescribed above. Results are shown as functions of the temperature inFIGS. 14 and 15. FIG. 14 is illustrative of the comparison betweenExamples 19 and 20 and Comparative Examples 1 and 2. FIG. 15 isillustrative of the comparison between Examples 21 and 22 andComparative Examples 1 and 2. According to FIGS. 14 and 15, it is clearthat the oxide ion conductivities of the oxide ion conductors ofExamples 19 to 22 are higher than those of the oxide ion conductors ofComparative Examples 1 and 2, when the temperature is not less than 500°C.

[0103] The results from above suggest that a fuel cell, which can beoperated at a middle range temperature of 500 to 700° C., can beconstructed by adopting an oxide ion conductor composed ofLa_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) (8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2) asa solid electrolyte.

[0104] As explained above, the crystals of the composite oxide, in whichthe value of (l+m) in La_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) is controlledto be 8 or more but less than 10, have the strained apatite typestructure. Therefore, the composite oxide exhibits excellent oxide ionconductivity even at a middle range temperature of 500 to 700° C.Therefore, the composite oxide (oxide ion conductor) as described abovecan be adopted, for example, as a preferred solid electrolyte for thefuel cell.

[0105] While the invention has been particularly shown and describedwith reference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. An oxide ion conductor composed of a compositeoxide containing: lanthanum, a tetravalent element A, and at least oneof a divalent or tetravalent element X with which said lanthanum issubstituted and a trivalent or pentavalent element Z with which saidelement A is substituted, wherein said composite oxide has a compositionformula which is represented by La_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p)provided that 8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2, and wherein said compositeoxide has a crystalline structure which belongs to an apatite typestructure.
 2. The oxide ion conductor according to claim 1, wherein saidelement A is Ge.
 3. The oxide ion conductor according to claim 1,wherein said element X is Sr or Zr, and said element Z is Al or P. 4.The oxide ion conductor according to claim 1, wherein a crystal systemof said crystal belongs to a hexagonal system, and a space group of saidcrystal is P6₃/m symbolized by a Hermann-Mauguin symbol.
 5. The oxideion conductor according to claim 1, wherein said element A is Ge, saidelement X is Sr or Zr, and said element Z is Al or P.
 6. The oxide ionconductor according to claim 5, wherein a crystal system of said crystalbelongs to a hexagonal system, and a space group of said crystal isP6₃/m symbolized by a Hermann-Mauguin symbol.
 7. A method of producingan oxide ion conductor, comprising the steps of: obtaining a mixedpowder in which a lanthanum compound, a compound of a tetravalentelement A, and at least one of a compound of a divalent or tetravalentelement X and a compound of a trivalent or pentavalent element Z aremixed in a ratio to produce La_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p)provided that 8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2; forming said mixed powderto obtain a formed product; and sintering said formed product to obtainsaid oxide ion conductor having a crystalline structure which belongs toan apatite type structure and having a composition formula which isrepresented by La_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that8≦l+m<10, 0≦m≦2, 0≦n≦2, 0≦p≦2.
 8. The method of producing said oxide ionconductor according to claim 7, wherein Ge is selected as said elementA, and said sintering is performed at a temperature of 1400 to 1700° C.9. The method of producing said oxide ion conductor according to claim8, wherein said sintering is performed at a temperature of 1450 to 1600°C.
 10. The method of producing said oxide ion conductor according toclaim 9, wherein said sintering is performed at a temperature of 1500°C.
 11. The method of producing said oxide ion conductor according toclaim 7, wherein said lanthanum compound, said compound of saidtetravalent element A, said compound of said divalent or tetravalentelement X, and said compound of said trivalent or pentavalent element Zare oxides.
 12. A method of producing an oxide ion conductor, comprisingthe steps of: obtaining a mixed powder in which a lanthanum compound, acompound of a tetravalent element A, and at least one of a compound of adivalent or tetravalent element X and a compound of a trivalent orpentavalent element Z are mixed in a ratio to produceLa_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that 8≦l+m<10, 0≦m≦2,0≦n≦2, 0≦p≦2; heating said mixed powder to obtain a granular material ofa composite oxide of lanthanum and germanium having a crystallinestructure which belongs to an apatite type structure and having acomposition formula which is represented byLa_(l)X_(m)(AO₄)_(6−n)(ZO₄)_(n)O_(p) provided that 8≦l+m<10, 0≦m≦2,0≦n≦2, 0≦p≦2; pulverizing said granular material to obtain a compositeoxide powder; forming said composite oxide powder to obtain a formedproduct; and sintering said formed product to obtain said oxide ionconductor composed of said composite oxide.
 13. The method of producingsaid oxide ion conductor according to claim 12, wherein said heatingstep for preparing said granular material is performed at a temperatureof 700 to 1200° C.
 14. The method of producing said oxide ion conductoraccording to claim 12, wherein Ge is selected as said element A, andsaid sintering is performed at a temperature of 1400 to 1700° C.
 15. Themethod of producing said oxide ion conductor according to claim 14,wherein said sintering is performed at a temperature of 1450 to 1600° C.16. The method of producing said oxide ion conductor according to claim15, wherein said sintering is performed at a temperature of 1500° C. 17.The method of producing said oxide ion conductor according to claim 12,wherein said lanthanum compound, said compound of said tetravalentelement A, said compound of said divalent or tetravalent element X, andsaid compound of said trivalent or pentavalent element Z are oxides.